1. Field of the Invention
The present invention relates to an electric power steering system that utilizes a worm gear system.
2. Description of the Related Art
Conventionally, an electric power steering system that utilizes a worm gear system as shown in FIG. 5 is known. The electric power steering system includes an upper shaft 103 that is connected to a steering shaft 100 interlocking with a steering wheel and a lower shaft 102 that is coaxially connected to the upper shaft 103 via a torsion bar 101. When the steering wheel is operated, a torque detection device 110 detects the torque that is applied to the steering wheel based on the magnitude of torsion of the torsion bar 101, an electric motor 120 for steering assist is driven and controlled, based on the detected torque, and the rotation of the electric motor 120 is transmitted to the lower shaft 102 through a speed reduction mechanism that includes a worm 130 and a worm wheel 131.
The torque detection device 110 is housed in a sensor housing 140, and the worm 130 and the worm wheel 131 are housed in a wows housing 150. The sensor housing 140 supports the upper shaft 103 and the lower shaft 102 with rolling bearings 104 and 105, and the worm housing 150 supports the lower shaft 102 with a rolling beating 106. The sensor housing 140 and the worm housing 150 are fitted in a socket joint manner and coupled through bolts (not shown) that fasten the sensor housing 140 and the worm housing 150. (See FIG. 5 of Japanese Patent Application Publication No. 2006-103636.)
As described above, in order that the sensor housing 140 and the wow housing 150 are fitted by means of the socket joint and then both housings are fastened with bolts, a gap is required for the portion to be fitted. Furthermore, in order to fasten both of the housings with bolts, a gap through which a bolt passes is required in a bolt insertion hole. Due to the gap in the fitting portion and the gap of the bolt insertion hole, when the sensor housing 140 and the worm housing 150 axe fastened, there is a possibility that slight relative displacement between both housings occurs.
When such the relative displacement occurs, the distance between rotating shafts of the worm 130 and the worm wheel 131 may change, and an engagement gap between the worm 130 and the worm wheel 131 may vary. For example, when the sensor housing 140 and the worm housing 150 are fastened in the state that the engagement gap becomes large, rattle noise occurs easily due to backlash being increased.